1. Field of the Invention
The present invention relates to a photocatalyst coated products and a process for coating with titania (TiO2) as a photocatalyst having a decomposing function including deodorizing, antibacterial and soil-resisting actions, and also a hydrophilic function, more particularly to a photocatalyst coated product comprising a titania layer formed on a surface of a product to be treated made of a metal, a ceramic or a mixture thereof by injecting a titanium or titanium alloy-containing powder against the surface of the product to be treated, and also to a method for producing a photocatalyst layer as a process for forming or coating the titania layer.
Photocatalysts containing as the major component titania having excellent decomposing and hydrophilic functions have been utilized conventionally in many fields. The decomposing function will now be described. Irradiation of ultraviolet light contained in the sunlight or fluorescent light upon titania causes generation of electrons and positive holes on the titania surface, and the electrons reduce the atmospheric oxygen into super oxide ions (O2), whereas the positive holes oxidize the moisture deposited on the titania surface into hydroxyl group radicals (OH). These superoxide ions and hydroxyl group radicals carry out oxidative decomposition of organic compounds including soil and the like present on the titania surface.
To describe now the hydrophilic function, the superoxide ions and hydroxyl groups generated by the ultraviolet irradiation as described above decompose hydrophobic molecules present on the titania surface to produce hydroxyl groups, and the atmospheric moisture is adsorbed by the hydroxyl groups thus produced to form a thin water film, thus imparting hydrophilicity to the titania surface. Accordingly, photocatalysts are frequently utilized in lenses, interior materials and furniture, such as mirrors, wall papers and curtains for the purpose of imparting deodorizing, antibacterial and soil-preventing actions, because of their hydrophilic function as well as the decomposing function.
When these photocatalytic functions are to be utilized in products such as interior materials and furniture, the product is impregnated with titania as a major component of the photocatalyst and is irradiated well with ultraviolet light. As a technique of achieving this, it is practiced to form a titania layer on the surface of a product to be treated.
2. Description of the Prior Art
As one method of forming a titania layer, a product to be treated made of titanium or titanium alloy is oxidized on the surface to form an oxide layer or a titania layer utilizing its liability to undergo oxidation reactions, since titanium per se is an active metal and have particularly great affinity with oxygen.
As other methods for forming a titania layer, a sol-gel method and a binder method are employed.
According to the sol-gel method, an organic titanium sol such as of titanium alkoxide and titanium chelates, which are precursors of titania, is applied onto the surface of a product to be treated having heat resistance, such as glass and ceramics by means of spray coating and the like, and then dried to cause gelation, followed by heating to 500xc2x0 C. or higher and form a stiff titania layer. Since titania particles are distributed over the entire surface of the product to be treated, the titania layer thus formed has high decomposition power and high hardness.
Meanwhile, according to the binder method, titania particles are immobilized on the surface of a product to be treated using a binder, for example, an inorganic binder such as silica or an organic binder such as silicone. The difference of the binder method from the sol-gel method is that the heating temperature may be the hardening temperature of the binder, so that the former requires a heat treatment at about 100xc2x0 C. or lower and no high-temperature treatment.
The conventional photocatalyst coating methods described above involve the following problems.
(1) The method of forming a titania layer by surface-oxidizing a product to be treated made of titanium or titanium alloy involves problems in that titanium per se is expensive to cause cost elevation and that titanium has poor processability and the fields of its application are limited.
(2) The sol-gel method also involves a problem in that it requires a heat treatment at about 500xc2x0 C. or higher for converting the organic titanium such as titanium alkoxide and titanium chelates, which are precursors of titania, into a titania layer, so that the product to be treated should have heat resistance, and that the product to be treated is limited to glass, ceramics and the like. If a titania layer is to be formed on the surface of a metal according to the sol-gel method, the metal surface is oxidized by the high-temperature heat treatment to cause reduction of commercial value due to deterioration and reduced luster.
The sol-gel method further involves a problem in that it requires much time and labor since the organic titanium is applied many times, that it requires an expensive equipment to cause cost elevation, and that harmful waste is by-produced.
(3) Meanwhile, the binder method can solve the problems inherent in the sol-gel method and enjoys merits in that it can treat various kinds of products and that it is relatively inexpensive. However, it involves a problem in that it is necessary to use as the binder a material which has high adhesion with the product to be treated and which is not susceptible to the decomposing function of the photocatalyst, and that selection of binder influences the effect of the catalyst.
Further, the titania layer formed according to the binder method has a hardness lower than that of the layer formed according to the sol-gel method, disadvantageously. In order to increase the hardness of the titania layer to be obtained according to the binder method, the amount of binder is increased to enhance adhesion. In this case, however, the amount of titania is reduced relative to the binder, and the titania layer shows poor decomposing power. On the contrary, if the amount of binder is reduced, the amount of titania to be exposed on the surface of the product to be treated is increased to show enhanced decomposing power, but the adhesion is reduced to readily cause peeling of the titania layer, resulting in the reduced hardness, disadvantageously.
The present invention was developed with a view to solving the problems described above, and it is an objective of the present invention to provide a photocatalyst coated product having excellent photocatalytic functions including the decomposing and hydrophilic functions which were imparted by forming by using a simple blasting treatment a titania powder as a photocatalyst having high hardness and high adhesion with products to be treated and a method for producing the photocatalyst layer.
In order to attain the above objective, the photocatalyst coated product according to the present invention comprises a titania layer formed on a surface of a product to be treated which is a metal product, a ceramic or a mixture of them; wherein a titanium or titanium alloy-containing powder is diffused, penetrated and oxidized into the surface of the product to be treated.
Meanwhile, the method for producing a photocatalyst layer as described above comprises injecting a titanium or titanium alloy-containing powder against a surface of a product to be treated which is a metal product, or against a ceramic or a mixture of them to effect diffusion of the titanium contained in the titanium or titanium alloy-containing powder over the surface of the product to be treated mentioned above and also oxidation of the titanium to form a titania layer.
Incidentally, the titanium or titanium alloy-containing powder is injected at an injection velocity of 80 m/sec or higher and under an injection pressure of 0.29 MPa or higher.
Further, the titanium or titanium alloy-containing powder has an average particle size of 20 to 800 xcexcm, preferably 30 to 300 xcexcm, and the shape of the powder, which may not be limited so long as it allows formation of titanium layer by the injection treatment, is preferably spherical or polygonal.
It should be noted here that in the present specification, the powder containing titanium as a major component is referred to as xe2x80x9cthe titanium or titanium alloy-containing powderxe2x80x9d and includes those which have reacted with the atmospheric oxygen to have stable oxide layers (TiO, Ti2O3, TiO2) formed on the surface thereon, respectively.
If a titanium or titanium alloy-containing powder (hereinafter referred to as xe2x80x9ctitanium powderxe2x80x9d) is injected at a high injection velocity against the surface of a product to be treated made of a metal, a ceramic or a mixture of them, a thermal energy is generated by the difference between the velocity of the powder before impingement upon the surface of the product to be treated and that after the impingement and in view of the law of energy invariability. Since this conversion of energy occurs only at modified portions brought about by the impingement of the titanium powder, localized temperature rise occurs in the titanium powder and near the surface of the product to be treated.
Meanwhile, since the temperature rise occurs proportional to the velocity of the titanium powder before the impingement, the temperature of the titanium powder and that of the surface of the product to be treated can be increased by increasing the injection velocity of the titanium powder. It is surmised here that since the titanium powder is heated on the surface of the product to be treated, the titanium contained in the titanium powder is activated to be adsorbed on the surface of the product to be treated, and also the titanium undergoes an oxidation reaction with the atmospheric oxygen to form a titania layer having photocatalytic functions on the surface of the product to be treated.
Thus, the photocatalyst coated product and the method for producing the photocatalyst layer according to the present invention, which are different from the conventional photocatalyst coating techniques, relate to a photocatalyst coated product having a titania layer formed on the surface of a product to be treated by diffusion and penetration of titanium into the product to be treated and by its oxidation reaction induced by the temperature rise occurred in the titanium powder and in the product to be treated when the titanium powder is impinged upon the product to be treated by a blasting treatment and to a method for producing the photocatalyst layer.
To describe more specifically, let us take, for example, cementation generally practiced. For example, when a metal product A is buried in a metal powder B to effect diffusion of the latter at a temperature t, the diffusion is carried out primarily by the metal vapor developed from the metal powder B or a metal halide vapor formed by a reaction of the metal powder with an additive, like carburization is carried out primarily by CO gas. Take carburization for example, there occurs no reaction between Fe contained in a ferrous metal product and CO by merely causing CO gas to deposit physically on the surface of the metal product as can be removed readily by external forces, heating or other physical methods. However, the CO gas is activated to be adsorbed on the Fe surface by application of heat or other energy in a predetermined quantity or more. The CO gas having undergone activated adsorption then undergoes thermal dissociation into carbon dioxide and carbon. The carbon formed in this reaction is supposed to diffuse into the Fe lattice to induce the carburization phenomenon.
In view of the above carburization phenomenon, according to the photocatalyst coating in the present invention, it can be considered that a titania layer is formed on the product to the treated through the following process.
For example, if a titanium powder is injected to impinge upon the surface of a product to be treated made of a metal, a ceramic or a mixture of them at an injection velocity of 80 m/sec or higher and under an injection pressure of 0.29 MPa or higher, the velocity of the titanium powder is reduced before and after the impingement. When the law of energy invariability is taken into consideration it can be surmised that a thermal energy is produced by the internal friction caused by deformation of the product to be treated occurred at the impinged portions, and the titanium powder is heated by this thermal energy on the surface of the product to be treated to effect activated adsorption of titanium by the product to be treated and diffusion into it, and that the titanium further reacts with the atmospheric oxygen to be oxidized and form a titania layer.
Incidentally, since the objective of the present invention is to effect both activated adsorption of titanium on the surface of the product to be treated and oxidation reaction of titanium utilizing the temperature rise of the titanium powder, it is essential to use a relatively small shot and not a heavy shot so that the titanium powder can be heated instantaneously by the thermal energy. Since titanium has a low density and relatively low thermal conductivity compared with other metals, localized concentration of heat is liable to occur, so that a powdery powder having a particle size of 800 xcexcm to 20 xcexcm, preferably 300 xcexcm to 30 xcexcm can be used.
The titanium powder desirably has a spherical or polygonal shape so as to carry out efficient heating of the titanium powder.
Further, titanium has good affinity with oxygen to form frequently an oxide layer on its surface and are stabilized. However, when a titanium power having such oxide layer is injected against the surface of a product to be treated as described above, the oxide layer is ruptured instantaneously by the impingement upon the product to be treated, and thus titanium is surmised to undergo activated adsorption on the surface of the product to be treated.